1. Technical Field
The present invention relates to a rotational recording apparatus, such as a hard disk drive (hereinafter referred to as an HDD), and a control method therefor, and in particular to a technique for applying an adaptive algorithm for the feed forward control wherein driving in a sine wave is employed for a head seek.
2. Description of the Related Art
For a rotational recording apparatus, especially for an HDD, a VCM (Voice Coil Motor) is employed to drive a head that is used to read or write data. An arm is moved in the radial direction of a disk upon receiving a VCM drive current, and as a result, the head provided at the distal end of the arm is moved in accordance with the direction and the value of a drive current. The drive current is generated by the feedback control or by the feedback and feed forward control, and the head is positioned above a predetermined track.
In order to quickly move the head to a target track, generally the head positioning is performed by combining multiple control modes. That is, when the seek distance for a target track is sufficiently long, the velocity control mode in which the head velocity is used as a state value is employed. When the distance for the target track is shortened, the mode is changed to the position control mode in which the head position is used as a state value. The position control mode is separated into a settling mode for moving the head to the range of the target track, and a track following mode for maintaining the head at a centered position along the target track.
When these control modes are changed, especially when the velocity control mode is changed to the position control mode, the drive current may be discontinuously changed, depending on the state. The drive current may also be discontinuously changed in the initial head driving stage. A discontinuous change of the drive current may cause the VCM to produce acoustic noise.
Therefore, in order to reduce acoustic noise, techniques have been devised for shaping drive currents to obtain sine waves. For example, in Japanese Unexamined Patent Publication No. 2000-123502 a technique is disclosed for moving a transducer (head) in accordance with a sine-wave acceleration trajectory during the execution of a seek routine (velocity control mode). Using this technique, the high frequency element of a drive current is reduced by obtaining the sine-wave acceleration trajectory, and acoustic noise can be minimized.
When the VCM drive current is to be shaped to obtain a sine wave, a difference between a target position and the current head position is detected, and in accordance with a feedback control wherein negative feedback is performed for a drive current to reduce the difference to zero, the head does not always follow an intended trajectory. FIG. 7 is a graph showing a target position and an actual head position attained by the feedback control discussed by the present inventor. The vertical axis represents a track (position), and the horizontal axis represents a sample. The sample indicates an arbitrary sampling time, and is converted into the time level by multiplying the sampling time (sampling cycle) T. A broken line indicates the trajectory of the target position, and a solid line indicates the trajectory of the actual head position. The approach of the trajectory of the head position to that of the one for the target position is delayed, and overshoots the target track (one track). As a result, the period of time required to reach the target track is extended. This is because there is a phase delay included in the transfer function of the feedback loop, and it is apparent that a quick movement of the head is difficult when only feedback control is employed. A countermeasure for increasing the gain of the feedback loop may be employed; however, since the drive current can not be calculated as the difference between the target position and the current position is reduced, matching the target position becomes increasingly difficult, and the probability of an overshoot is increased.
Therefore, a countermeasure involving the combining of the feed forward control processes may be employed. However, to carry out the feed forward control, what will be the system response to the input of data must be understood, i.e., a drive current that is to be input and how this current will move the head must be understood, and this Is requires system modeling. System modeling, however, is not easy, because when apparatuses are mass produced, manufacturing variances may cause affected units to function slightly differently, making it difficult to obtain in advance a model that will be appropriate for all such products. Further, even when actual measurements are employed to provide a useful model for a specific apparatus, it is substantially impossible to obtain models for all possible product variations using measurements.
Therefore, an algorithm for adaptively updating a system parameter, such as an adaptive algorithm, can be applied for a feed forward system. However, the stability of the system using the adaptive algorithm is not guaranteed unless the system has been devised.
Thus there is a need for a technique for adaptively and stably updating the feed forward value required to drive the head of a rotational recording apparatus along a sine-wave trajectory. According to such a technique, the moving head trajectory can match the target trajectory, and the drive current can be shaped so that, for the reduction of acoustic noise, it more nearly corresponds to the sine wave.